This invention relates to thermosetting resin compositions having excellent heat resistance and fast-curing properties.
There are a variety of heat-resistant resins which can be used as insulating materials in order to meet the demands for greater capacities, smaller sizes, lighter weights, and higher degrees of reliability of electronic devices and equipments as well as for better heat resistance characteristics, longer service lives, and maintenance-free properties of electrical machinery and apparatus. Among these heat-resistant resins, imide resins of the addition polymerization type, such as bismaleimide resins and aromatic diamine-modified bismaleimide resins, are well known. Such bismaleimide resins have excellent heat resistance, but are not very satisfactory because of the difficulties in processing. The melting points of bismaleimides are high and their curing rates are very low. Moreover, their poor solubility in organic solvents necessitates the use of polar solvents, such as N-methyl-2-pyrrolidone and dimethylformamide, which are undesirable from the viewpoint of environmental pollution. Furthermore, an organic peroxide such as dicumyl peroxide or benzoyl peroxide is usually added to maleimide resins for the purpose of promoting their cure. When such a resin composition is used as impregnating varnish to make prepregs, the cleavage products of the organic peroxide remaining in the prepregs may lead to a reduction in quality and performance of the finally obtained laminates and, especially in the case of copper-clad laminates, to blistering or peeling-off of the copper foil.
It is well known that bismaleimides form a three-dimensional network when they are heated above their melting points or subjected to the action of free radicals. It is also known that the presence of a catalytic amount of a free-radical initiator is desirable for the purpose of achieving an effective crosslinking reaction (see, for example, J. Amer. Chem. Soc., Vol. 81, p. 1187, 1959). By nature, many free-radical initiators have a low activation energy for their decomposition and are unstable and easily decomposable even at room temperature, so that special care must be taken in their storage and handling.